diff --git a/src/dmclock/sim/src/sim_recs.h b/src/dmclock/sim/src/sim_recs.h
index b64750db4af..759ab4e1413 100644
--- a/src/dmclock/sim/src/sim_recs.h
+++ b/src/dmclock/sim/src/sim_recs.h
@@ -21,6 +21,7 @@
 #include <string>
 #include <mutex>
 #include <iostream>
+#include <functional>
 
 
 using ClientId = uint;